1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and a process cartridge therefor. In particular, the present invention relates to an electrophotographic image forming apparatus using a proximity charger which charges a photoreceptor while a narrow gap is formed between the photoreceptor and the charger, and a process cartridge therefor.
2. Discussion of the Background
Recently the growth of electrophotographic image forming apparatus using a photoreceptor, such as copiers, printers and facsimiles, is remarkable. In particular, photo-printers capable of recording digital information using light have been drastically improving in recording qualities and reliability. This digital recording technique is applied to copiers as well as photo-printers. The copiers to which the conventional analogue copying technique and this digital technique are applied have various image forming functions. Therefore it is considered that the demand for such copiers increases more and more.
In attempting to reduce the quantity of ozone and NOx generated in an electrophotographic image forming apparatus and the electric power consumption of the image forming apparatus when performing charging, charging methods using a charging roller have been proposed.
For example, Japanese Laid-Open Patent Publication No. (hereinafter referred to as JOP) 4-336556 discloses a contact charging device in which a charging roller charges a photoreceptor while contacting the photoreceptor. In the contact charging device, the surface of the charging roller is made of a dielectric material, and the rotating direction of the charging roller is the same as that of the photoreceptor (i.e., at the contact point between the charger and the photoreceptor, the moving direction of the charger is opposite to that of the photoreceptor).
In this case, even when there is a pinhole on the surface of the photoreceptor, a problem in that a charge is not formed around the pin hole does not occur. This is because the surface of the charging roller is dielectric and therefore the charges of the portion of the charging roller to be contacted with the pinhole of the photoreceptor do not decay when performing charging. In addition, even when the photoreceptor and dielectric charging roller are frictionally charged due to friction between the photoreceptor and the charging roller at the contact point thereof, a surface area of the photoreceptor to be charged can be contacted with a surface area of the charging roller having a relatively low charge potential (i.e., a surface area of the charging roller which is not a surface area having a high potential due to rubbing with the photoreceptor), and thereby the photoreceptor can be charged to a desired potential even at a relatively low applied voltage. Since the charging roller charges the photoreceptor while contacting the photoreceptor, the applied voltage is relatively low compared to non-contact chargers such as scorotrons, and therefore the quantity of the above-mentioned reactive gasses generated, such as ozone and NOx, can be reduced.
However, the contact charging devices have the following drawbacks:
(1) uneven charging (i.e., traces of a charging roller) due to uneven contact of the charging roller with the photoreceptor, etc.;
(2) large charging noise;
(3) charging ability deteriorates when toner particles, etc. present on the surface of the photoreceptor adhere on the surface of the charging roller;
(4) photosensitive properties of the photoreceptor change when one or more constituents of the charging roller adhere (migrate) to the photoreceptor; and
(5) the charging roller deforms when the photoreceptor is stopped for a long period of time, resulting in uneven charging.
The uneven charging mentioned above in item (1) is caused by adhesion of the constituents of the charging roller, which are migrated from the charging roller, on the photoreceptor when the photoreceptor is stopped. The large noise mentioned above in item (2) is caused by vibrational contact of the charging roller with the photoreceptor. The vibration of a charging roller is caused when an AC voltage is applied to the charging roller.
In attempting to solve these problems, proximity charging devices have been proposed. In the proximity charging devices, a photoreceptor is charged by applying a voltage to a charger, which is arranged such that a narrow gap of from 0.005 to 0.3 mm is formed between the charger and the photoreceptor.
The proximity charging devices do not cause the problems mentioned above in items (4) and (5) because the charger does not contact the photoreceptor. In addition, with respect to the problem mentioned above in item (3), the proximity charging devices are superior to the contact charging devices because the quantity of toner particles adhered on the charger is less than in the case of contact charging devices.
Various proximity charging methods have been disclosed in, for example, JOPs 2-148059, 5-127496, 5-273837, 5-307279, 6-308807, 8-202126, 9-171282 and 10-288881.
These publications relate to proximity charging methods and it is described therein that a photoreceptor is experimentally charged with a charger while a gap is formed therebetween to observe whether the photoreceptor is evenly charged. However, there is no description in the publications as to how the charger is set closely to the photoreceptor, namely, only ideas of construction of proximity chargers are described therein. Indeed, it is not easy to form a uniform gap not greater than hundreds of micrometers between a charger and a photoreceptor and stably maintain the gap. Namely, proximity charging methods have a big problem of how to stably maintain a gap not greater than hundreds of micrometers between a charger and a photoreceptor.
To the contrary, specific examples of how to set a charger closely to a photoreceptor are described in JOPs 5-107871, 5-273873, 7-168417 and 11-95523.
JOPs 5-107871 and 5-273873 have proposed a method in which an insulating tape whose ends are fixed by springs and which serves as a gap forming member is set between a charger and a photoreceptor, to form a gap between the charger and the photoreceptor. This method is effective in forming a gap between a photoreceptor and a charger. However, when such a gap forming member is practically set on an image forming apparatus, a tension is applied to the springs in only one direction because the photoreceptor rotates in only one direction. Therefore, the springs are easily fatigued. In addition, when such a member is set in the image forming apparatus, the construction of the resultant image forming apparatus becomes complex although this member has a simple mechanism. Therefore the maintenance of the image forming apparatus cannot be easily performed. For example, the image forming apparatus has a drawback in that when the gap forming member is changed, the photoreceptor has to be also changed.
JOP 7-168417 discloses a method in which a gap is formed between a photoreceptor and a charger by setting spacers on bearings of a charger, wherein the spacers contact the surface of the photoreceptor. In this case, the spacers have to be different from the charging portion of the charger in size and material, resulting in complication of the construction of the charger. In addition, in this case the charging roller is made of an insulating material, and therefore a voltage applying roller is needed, resulting in further complication of the construction of the charger and increase of manufacturing costs of the charger.
JOP 11-95523 discloses a method in which a gap is formed between a charger and a photoreceptor by setting a gap forming member on at least one of the charger and the photoreceptor. This apparatus has a simple consruction, but there is no description about the construction of the gap forming member and how to set the gap forming member. Therefore, it is unknown whether a gap can be stably maintained (i.e., the photoreceptor can be stably charged) for a long period of time.
JOP 4-360167 discloses a proximity charging device using a charger, on both ends of which a projected portion is formed to form a gap between the charger and a photoreceptor. By charging the photoreceptor with this charger while contacting the charger with the photoreceptor, proximity charging can be performed. However, there is no description about how to support the gap forming member and the photoreceptor and how to arrange the gap forming member and the photoreceptor. Therefore, it is unknown whether a gap can be stably maintained (i.e., the photoreceptor can be stably charged) for a long period of time.
In addition, there is no description about the measures against uneven charging around the edge portions of the photoreceptor closely to the projected portions. Further there is no description about the measures against accumulation of toner particles on the edge portions of the photoreceptor closely to the projected portions when the charger is repeatedly used. Therefore, it is unknown whether this proximity charging device can be stably used for a long period of time. Namely, the reliability of this proximity charging device is unknown in particular when the charging device is practically used repeatedly.
JOP 7-121002 discloses an image forming apparatus in which a ring-shaped spacer is set on both ends of a cylindrical photoreceptor to form a gap between the photoreceptor and a charger. Around the photoreceptor, other devices such as an image developer, an image transferer and a cleaner are set while contacting the photoreceptor or being closely to the photoreceptor. When such a ring spacer is set on both ends of the photoreceptor, such devices cannot be provided on the ring spacer. Therefore the length of the photoreceptor in the axial direction needs to be extended to secure the desired image forming portion on the photoreceptor.
In addition, in this charging method charging near the ring spacers tends to become uneven (i.e., the potential on the edge portions tends to decrease). When such a charging method is used in combination with a nega-posi developing method which is suitable for digital image writing because the image writing time can be saved, a problem such that background development is observed in these edge portions of the photoreceptor tends to occur.
Further, the spacers themselves and/or the charger tend to be contaminated. Therefore, the edge portions of the photoreceptor neat the spacers should be cleaned such that there is no residual toner particles. However, since the spacers are formed on the photoreceptor, the edge portions cannot be cleaned. Accordingly, it is considered that this charging device has poor reliability when practically used repeatedly.
Because of these reasons, a need exists for a proximity charging device which has a simple construction and in which a gap is stably maintained between the charger and a photoreceptor even when repeatedly used.
Accordingly, an object of the present invention is to provide a simple and low-cost proximity charging device in which the above-mentioned drawbacks of the contact charging methods can be remedied and which can be practically used. Specifically, a gap can be stably maintained between the charger and a photoreceptor without forming a toner film on the surface of the charger even when the charging device is repeatedly used.
Another object of the present invention is to provide a proximity charging device which does not cause uneven charging specific to proximity charging even when used for a long period of time, resulting in formation of good images for a long period of time.
Yet another object of the present invention is to provide an electrophotographic image forming apparatus and a process cartridge therefor, by which images having good image qualities can be stably produced even when repeatedly used without frequently changing the photoreceptor.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophotographic image forming apparatus including at least an image bearing device including a photoreceptor including an electroconductive substrate, a photosensitive layer on the substrate and optionally a protective layer on the photosensitive layer and which rotates in a direction, wherein the photoreceptor has an image forming portion having two ends substantially parallel to the rotating direction; a charging roller which has a gap forming member on both ends thereof to form a gap between the surface of the image forming portion of the photoreceptor and the periphery surface of the charging roller and which is configured to charge the photoreceptor while rotating, wherein the gap forming members do not contact the image forming portion of the photoreceptor; a light irradiator configured to irradiate the photoreceptor with light to form an electrostatic latent image in the image forming portion of the photoreceptor; an image developer configured to develop the latent image with a toner to form a toner image thereon; and an image transferer configured to transfer the toner image onto a receiving material, wherein the following relationship is satisfied:
txe2x89xa72g 
wherein g represents the gap and t represents a distance between the inside edge of one of the gap forming members and one of the two ends of the image forming portion of the photoreceptor, which is closer to the one of the inside edge of the one of the gap forming members.
The gap is preferably from 10 to 200 xcexcm.
The gap forming members can be formed by coating a coating liquid; winding a tape, etc.; cutting the central portion of the surface layer of the charging roller; or the like method.
Preferably the gap forming members contact non-image portions formed on both ends of the photoreceptor, or flanges set on both ends of the photoreceptor.
Alternatively, the charger may have a projected portion on both ends thereof, which serves as the gap forming member.
The gap forming member may be a combination of the flange formed on both ends of the photoreceptor and the projected portion of the both ends of the charger.
The photoreceptor may be a belt-shaped photoreceptor which is supported and driven by at least a driving (or driven) roller. In this case, the width of the roller is longer than that of the belt photoreceptor, and the extended portions of the roller contacts the gap forming members to form a gap. In this case, the gap forming member may be a projected portion of the charger.
It is preferable that at least one of the charger and the photoreceptor (or the driving or driven roller) is pressed toward the other by a spring, etc.
In addition, it is preferable that the charging roller and the photoreceptor have a respective driving device such as gears, couplings and belts so as to be independently rotated.
In another aspect of the present invention, a process cartridge is provided which includes at least a photoreceptor including an electroconductive substrate, a photosensitive layer on the substrate and optionally a protective layer on the photosensitive layer and which rotates in a direction, wherein the photoreceptor has an image forming portion having two ends substantially parallel to the rotating direction; and a charging roller which has a gap forming member on both ends thereof to form a gap between the surface of the image forming portion of the photoreceptor and the periphery surface of the charging roller and which is configured to charge the photoreceptor while rotating, wherein the gap forming members do not contact the image forming portion of the photoreceptor; a light irradiator configured to irradiate the photoreceptor with light to form an electrostatic latent image in the image forming portion of the photoreceptor, wherein the following relationship is satisfied:
txe2x89xa72g 
wherein g represents the gap and t represents a distance between the inside edge of one of the gap forming members and one of the two ends of the image forming portion of the photoreceptor, which is closer to the one of the inside edge of the one of the gap forming members.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.